Automatic gain control circuit



May 24, 1938. w KOCH AUTOMATIC GAIN CONTROL CIRCUIT Original Filed May8, 1934 KOCH W Li W MM WUUE c vx INVENTOR WINFIELD R BY ATTORNEYPatented May 24, 1938 UNlTED s'rsrss Z,ll8,28l rarest crease AUTOMATICGAIN CONTROL CIRCUIT Winfield R. Koch, Merchantville, N. J., assignor toRadio Corporation of America, a corporation of Delaware Originalapplication May 8, 1934, Serial No. 724,496. Divided and thisapplication June 20, 1936, Serial No. 86,224.

'7 Claims.

V 1934, Patent No. 2,093,565, September 21, 1937.

It often happens that the control bias voltage obtainable from thevolume control tube of a radio receiver is not sufficient to keep theaudio output characteristic fiat for all signal inputs. For example, inthe case wherein a diode is utilized as the volume control tube, theaforementioned type of flat characteristic may be somewhat difficult toobtain. It may therefore be stated that it is one of the main objects ofthe present invention to provide an automatic volume control circuit fora radio receiver, the circuit amplifying the control bias Voltage andadding the amplified voltage to the original control bias, so that thesum of the two bias voltages are effective in controlling theamplification of the controlled tubes of the receiver.

While amplification of a volume control bias voltage has been consideredin the past, additional direct current energy sources have been foundnecessary to operate the control voltage amplifier. This is noteconomical in compact receivers, or receivers of the A. C.D. C. type.Hence, it may be stated that it is an additional, and important, objectof the present invention to utilize the voltage drop in the grid leak ofthe local oscillator network of a superheterodyne receiver forfurnishing the direct current voltages required to secure amplificationof the automatic volume control bias voltage.

Still another object of the invention is to provide a radio receiver ofthe superheterodyne type, the receiver utilizing a composite localoscillator-first detector tube, and the voltage drop developed acrossthe grid leak of the local oscillator portion of the composite tubebeing used to energize a portion of the automatic volume control.network which functions to amplify control bias voltage developed by thesaid volume control network.

Still other objects of the invention are to improve generally automaticvolume control circuits for radio receivers, and more especially toprovide such circuits which are not only reliable in operation, buteconomically manufactured and assembled in radio receivers.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,will best be understood by reference to the following description, takenin connection with the drawing, in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawing:

Fig. l diagrammatically rep-resents a superheterodyne receiver embodyingthe invention,

Fig. 2 schematically shows a modified form of the invention.

Referring now to the accompanying drawing, wherein like referencecharacters in the different figures denote similar circuit elements,there is shown in Fig. 1 a superheterodyne receiver of a conventionaltype. This receiver includes the usual signal collector I, which may bea grounded antenna circuit, and a tunable radio frequency amplifier 2having its tunable input circuit coupled to the signal collector. Thenumeral 3 denotes a composite local oscillator-first detector tube ofthe pentagrid type. The tube 3 and its associated tunable circuits hasbeen fully de scribed by, and is claimed in, application Serial No.654,421, filed January 31, 1933, in the name of J. C. Smith.

Briefly, the tube 3, of the 2A7 type, comprises a cathode, a plate, asignal grid, an oscillator grid, an oscillator anode, and a pair ofscreen grids between which is disposed the signal grid. The tunablelocal oscillator circuit includes a coil 4 and variable tuning condenser5, one side of the condenser being grounded, and the grounded side ofthe condenser being connected to the low alternating voltage side ofcoil it through a condenser 6. The ungrounded sides of coil 6 andcondenser 5 are connected to the oscillator grid 6 of tube 3, and theoscillator anode l. is connected to a source of positive potential ll(not shown) through a coil 8 which is magnetically coupled with the coil4. The two screen grids, between which the signal grid is disposed, areconnected to a point of positive potential S.

The plate of tube 3 is connected to a point of positive potential B, theresonant network 9 tuned to the operating intermediate frequency beingconnected in the plate circuit of tube 3. A radio frequency by-passcondenser H3 is connected between the plate circuit lead tube andground, while the circuit 9 is magnetically coupled to the resonantinput circuit of the following I. F. amplifier IZ; the circuit l l,also,being tuned to the intermediate frequency. The condenser rotors of thetunable signal input circuits of amplifier 2, and tube 3 aremechanically coupled, and the rotors of condenser 5 are alsomechanically coupled to the afore-mentioned rotors, this being denotedby the dotted lines Hi.

It will be understood that the reference characters O, S and B maydenote appropriate voltage points on a common voltage supplypotentiometer, and such a common supply potentiometer would also be usedto feed the various electrodes of the different tubes of the system, thepotentiometer being omitted in order to preserve simplicity ofdescription.

The intermediate frequency amplifier 52 may be of the conventional type,and its output is coupled to the input electrodes of the followingsecond detector tube I3 which is of the pentode type. The couplingbetween the amplifier i2 and second detector !3 is provided. by atransformer [4 whose primary and secondary are tuned to the operatingintermediate frequency, and it will be noted that the present inventionis not limited to the utilization of only a single stage of intermediatefrequency amplification, but more than one stage may be utilized. One ormore stages of audio amplification may follow the second detector i3,and a reproducer of any desired type will terminate the receivingsystem. The circuit details following the tube is are not shown, becausethose skilled in the art are well acquainted with such details.

The automatic volume control system employed to maintain the signalintensity level to the second detector substantially constant while thereceiver is in operation will now be described. The numeral i5 denotesthe automatic volume control tube, and it will be observed that thistube is of the 55 type and embodied, in addition to a oath" ode, controlgrid and plate, a pair of diode anodes disposed outside the electronstream flowing from the cathode to the plate. Such a tube and itsproperties are well known to those skilled in the art at the presenttime, and it will sufiice to state that the two diode anodes areconnected together, a lead It connecting them to the signal grid circuitof second detector tube iii. The low alternating voltage side of thesignal input circuit of tube 33 is connected to the cathode of themulti-function tube 85 through a path which includes resistor R1 andlead H, the cathode of tube l5 being grounded through a condenser H8.

The control grid of tube i5 is connected by a lead I9 to the negativedirect voltage side of resistor R1, and condenser 28 is connected acrossthe last-named resistor. The plate of tube i5 is connected to ground;and the cathode of the tube is additionally connected to the groundedside of the local oscillator tuning condenser 5 through a path whichincludes lead 2!, resistor R2 and resistor R3. The low alternatingvoltage side of coil 4 is connected to point A intermediate theresistors R2 and R3. The volume control bias voltages are applied to thesignal grids of amplifier 2, tube 3 and amplifier 82 through a pathwhich includes lead 22, and branch leads 23, 24 and 25. Each of theseleads includes appropriate filter resistors, the lead 22 being connectedto the negative side of resistor R1, and each of the branch leads 23, 24and 25 being connected to the grid circuits of the controlled stages.Reference letters AVC are used to denote the automatic volume controlpath.

The resistor R3 is so chosen that the point A is' normally about 30volts negative with respect to ground due to the grid current flow ofthe local oscillator portion of the tube 3. For conditions of weaksignal there will be a very small voltage drop across the resistor R1,and it will be noted that the resistor is included in the diode circuitconnected between the diode anodes and the cathode of tube i5. Since thegrid of tube 15 is connected to the negative side of resistor R1, thebias on the grid will be relatively small when little or no signalenergy is impressed upon the input circuit of detector it. At suchminimum bias on the grid of tube 55 the plate impedance of the tube isrelatively low compared with resistor R2. The voltage drop acrossresistor R2, plus the voltage between cathode and anode of tube I 5,must equal the voltage across resistor R3. Because the tube resistanceis much smaller than .R2, the cathode-anode voltage will be small, and

the cathode will be at only a small negative voltage to ground. The A.V. C. voltage will be this voltage drop in the tube plus the Voltagedrop in R1. Both of these are small, and the A. V. C. voltage will besmall.

When a strong signal is tuned in the increased voltage drop acrossresistor R1 results in an increased bias on the grid of tube 85, so thatthis plate impedance is high compared with resistors R2 and R3. The biasfor the controlled tubes is then equal to substantially the voltage dropacross resistor R3 plus the voltage drop across resistor R1. It will,therefore, be seen that the control bias for increasing signal intensityis increased many times as much for the same signal level increase aswhen only the diode is used as a control mechanism. The controlledcharacteristic will, therefore, be much more complete and flat.

V The voltage drop across the grid resistor oi the local oscillationportion of tube 3 may also be employed for automatic volume controlusing a circuit as shown in Fig. 2; In this modification only thosecircuit elements are shown which are essential to an understanding ofthe modified portions of the circuit. Thus the automatic volume controltube in this case is a pentode tube 38. The pentode tub-e gives a morecomplete cut-off, and works with lower applied signal voltages and lowertube Voltages. The signal grid of the tube 30 is connected by lead 3iand condenser 32 to the high signal voltage side of the input circuit oftube i3, while the cathode of the tube tii'is connected by lead 33 to anintermediate point on resistor R3. The point A of resistor R3 isconnected to the cathode of tube 30 through a path which includesresistor R2, lead 3% and condenser 35, the lead 3! being connected tolead 3% and condenser 35 through a resistor 36. ihe screen grid of tube3 is grounded, and also is connected to the automatic volume controllead 3? through a condense 38'. Resistor R2 and condenser 35 act as afilter to prevent any small radio frequency voltages existing across R3from reaching the grid of tube 30.

For ordinary usage the tube 38 may be a 5? type tube with its plateconnected to ground through a resistor. The pentode construction permitsthe plate voltage to become much less than the screen voltage withoutaffecting the tube eiiiciency. If a delay action is desired, the bias onthe tube can be made more than the amount required for cutoff. A secondmethod of getting a delay action is to connect the plate resistor oftube 38 to a point which is positive with respect to ground, and uses adiode 39 to prevent the bias on the controlled tubes from becomingpositive. Of course, the grid currents of the controlled tubes may bemade to keep the control bias from becoming positive, instead of usingthe diode 39.

The circuit shown in Fig. 2 operates in the following manner; when nosignal is tuned in, tube 3D is biased substantially to cut-oi? by theposition of the tap on resistor R3. The voltage drop in resistor 38 dueto plate current of tube 38 will be small. The A. V. C. voltage tends tobecome positive with respect to ground, but current flowing through thediode 39 causes a voltage drop in resistors 31 and 38, and thus preventsthe A. V. C. voltage from becoming positive to any substantial value. Asthe signal is tuned in, the I. F. voltage is impressed on the grid oftube 30, causing an increase in average plate current, and an increasein voltage drop across resistor 38.

Because of the amplification in tube 30, the drop in the plate resistor38 will be several times as large as the I. F. voltage applied to thegrid. When the drop in resistor 38 becomes large enough, the plate ofthe tube will become negative with respect to ground, the currentthrough diode 39 will stop, and the A. V. C. voltage will becomenegative, biasing ofi the amplifier tubes. The action of the A. V. C.system is, therefore, such that no increase in A. V. C. voltage occursuntil the signal exceeds a predetermined value, but increases rapidlyfor stronger signals.

While I have indicated and described several systems for carrying myinvention into efiect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made, withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In a radio receiver provided with a local oscillator including aresistor in its grid circuit, an amplifier whose gain is to becontrolled, a rectifier, tube having grid and cathode input electrodesand a plate and including means for impressing signals upon its inputelectrodes, direct current voltage connections from said resistor to thegrid and cathode of the rectifier, said connections applying to saidgrid and cathode a no-signal cutofi bias developed by the flow ofoscillator grid current through said resistor, a gain control connectionfrom the plate circuit of the rectifier to said amplifier and a diodeconnected to the control connection for delaying the gain control of theamplifier.

2. In a radio receiver provided with a local oscillator including aresistor in its grid circuit, an amplifier whose gain is to becontrolled, a rectifier tube having grid and cathode input electrodesand a plate and including means for impressing signals upon its inputelectrodes, direct current voltage connections from said resistor to thegrid and cathode of the rectifier, said resistor developing a voltageapplied through said connections as a rectifier no-signal cut-off bias,a gain control connection from the plate circuit of the rectifier tosaid amplifier, the direct current connection to the rectifier cathodebeing from an intermediate tap on the said resistor and a diodeconnected to the control connection for delaying the gain control of theamplifier.

3. In a wave transmission system, a wave trans mission tube, anautomatic gain control network for the tube comprising a space dischargedevice, a resistor and a source of direct current voltage all arrangedin a series circuit, a direct current voltage connection between a gaincontrol electrode of the tube and a point on the resistor, meansmaintaining the internal impedance of said device sufiiciently high tosubstantially prevent current fiow through said resistor, and additionalmeans responsive to an increase in wave amplitude for decreasing themagnitude of said impedance whereby a direct current voltage isdeveloped across said resistor to serve as a biasing voltage for saidgain control electrode and means operatively associated with saidresistor for delaying the impression of said biasing voltage on the gaincontrol electrode ifor waves of less than a desired amplitude.

4. In a wave transmission system, a wave transmission tube, an automaticgain control network for the tube comprising a space discharge device, aresistor and a source of direct current voltage all arranged in a seriescircuit, a direct current voltage connection between a gain controlelectrode of the tube and a point on the resistor, means maintaining theinternal impedance of said device sufiiciently high to substantiallyprevent current fiow through said resistor, additional means responsiveto an increase in wave amplitude for decreasing the magnitude of saidimpedance whereby a direct current voltage is developed across saidresistor to serve as a biasing voltage for said gain control electrode,and a diode device connected to said resistor point for delaying theimpression of said biasing voltage on the gain control electrode forwaves having less than a desired amplitude.

5. In a wave transmission system, a wave transmission tube, a source ofbiasing voltage for the input electrodes of the tube which comprises aspace discharge device having a fixed resistor connected in series withits space current path, said series path including a source of directcurrent voltage, direct current voltage connections between said inputelectrodes and said device, and means for decreasing the internalimpedance of said device in accordance with wave amplitude increasethereby to increase the bias voltage applied to said input electrodesand additional means for delaying the impression of said bias voltage onsaid input electrodes for wave amplitudes of less than a predeterminedvalue.

6. In combination in a superheterodyne receiver, provided with a localoscillator network, said network including a tube having a grid leakresistor, an automatic gain control network for at least one signaltransmission tube of said receiver, said network comprising an electrondischarge tube provided with at least a cathode, a control electrode anda plate, means utilizing direct current voltage developed across saidleak resistor by oscillator grid current flow for biasing said controlelectrode, in the absence of received signals, in a sense tosubstantially prevent space current fiow through said control networktube, a resistor disposed in the space current path of said controltube, means for connecting a gain control electrode of said onetransmission tube to a point on said resistor which assumes a negativedirect current potential when signals are received, and means forimpressing received signals on the control electrode of said controlnetwork tube.

7. In a signal wave receiver, a signal transmission tube arranged tofeed signals to a demodulator, an automatic gain control circuitcomprising a tube having a resistor in its space current path, meansbiasing said control tube in a sense to reduce the space current fiowthrough the resistor in the absence of signal waves, means forimpressing signal waves on said control tube, a gain control connectionbetween a gain control electrode of said transmission tube and a pointof said resistor, a conductive diode including said resistor in itsspace current path and developing a direct current voltage thereacrosswhich is applied to said gain control electrode in a negative sense, andthe anode of said diode being connected to a point on said resistorsuchthat the diode is rendered non-conductive when the space current ofsaid control tube is increased by a signal wave of a predeterminedamplitude.

WINFIELD R. KOCH.

